Hi,
Carbon Brushes are made specifically for the motor that they are being used in and come in different degrees of hardness.
Carbon brush grades are only designed to carry a specific current. If you increase the current density then you increase the temperature at which the carbon brushes operate. If your current density is too low then you get rapid brush wear. Taken to extreme, this can mean that your brushes could either dust or wear out in days.
So cheap, incorrect brushes can wear out or damage the commutator if they are too hard.
Make sure you buy from the motor manufacturer or a known brand equivalent.
Martin